Importin beta can mediate the nuclear import of an arginine-rich nuclear localization signal in the absence of importin alpha

The import of proteins into the nucleus is dependent on cis-acting targetin g sequences, nuclear localization signals (NLSs), and members of the nuclea r transport receptor (importin-beta-like) superfamily. The most extensively characterized import pathway, often termed the classical pathway, is utili zed by many basic-type (lysine-rich) NLSs and requires an additional compon ent, importin alpha, to serve as a bridge between the NLS and the import re ceptor importin beta. More recently, it has become clear that a variety of proteins enter the nucleus via alternative import receptors and that their NLSs bind directly to those receptors. By using the digitonin-permeabilized cell system for protein import in vitro, we have defined the import pathwa y for the Rex protein of human T-cell leukemia virus type 1. Interestingly, the arginine-rich NLS of Rex uses importin beta for import but does so by a mechanism that is importin alpha independent. Based on the ability of the Rex NLS to inhibit the import of the lysine-rich NLS of T antigen and of b oth NLSs to be inhibited by the domain of importin or that binds importin b eta (the IBB domain), we infer that the Res NLS interacts with importin bet a directly. In addition, and in keeping with other receptor-mediated nuclea r import pathways, Rex import is dependent on the integrity of the Ran GTPa se cycle. Based on these results, we suggest that importin beta can mediate the nuclear import of arginine-rich NLSs directly, or lysine-rich NLSs thr ough the action of importin alpha.